To improve performance of photoelectric conversion devices such as solar batteries, it is important to efficiently take sunlight inside substrates that constitute the solar batteries. Therefore, texture processing is performed on a light-incident-side substrate surface to allow light reflected by the surface to enter again the on surface, so that much sunlight is taken inside the substrate for improving the photoelectric conversion efficiency. In the texture processing, fine concaves and convexes having a size of several tens of nanometers to several tens of micrometers are intentionally formed in the substrate surface.
A pyramid shape or an inverted pyramid shape is known as a representative example of a texture shape. The pyramid shape is a shape obtained by placing a quadrangular pyramid on a plane, and the inverted pyramid shape is a concavity shape that a quadrangular pyramid placed facedown is dug into the plane. When inverted pyramid textures are arranged on a substrate without spaces therebetween, the entire substrate surface is constituted by inclined surfaces of inverted pyramids. Almost all of reflected light at an inclined surface enters again an adjacent inclined surface and thus the light use efficiency is maximized.
To form such an inverted pyramid shape, the dependency of a silicon etching rate with respect to an alkaline aqueous solution on surface orientation is utilized. That is, when crystal silicon is etched by an alkaline aqueous solution, its etching rate is high for <100> crystal surface orientation and low for <111> crystal surface orientation. When a crystal silicon substrate whose surface has a <100> crystal surface orientation is etched by an alkaline aqueous solution, the etching rate is significantly reduced at a time when etching proceeds to a certain degree and a <111> surface is exposed. Consequently, a pyramid shape or an inverted pyramid shape formed of the <111> surface is formed.
However, according to this method, the size and arrangement of pyramid-shaped or inverted pyramid-shaped fine concaves and convexes become random and the reflectance cannot be necessarily suppressed to a minimized one. To solve this problem, an etching mask patterned in advance is formed on a substrate and an etching is performed. Consequently, fine concaves and convexes having a desired size can be formed at desired positions. In such a method, a resist can be used as the etching mask and a photolithography technique can be used as a patterning method. A method of using a silicon nitride film as the etching mask and laser processing as the patterning method has been proposed as another method (see, for example, Patent Literature 1).
This method is explained below. A silicon substrate is etched by using an acid or alkaline solution to remove a damaged layer at the time of slicing. Subsequently, an SiNx film (silicon nitride film) is formed as an etching mask by PECVD (plasma-enhanced chemical vapor deposition). A pattern opening is formed in this etching mask by a laser. The silicon substrate is etched by using an acid or alkaline solution to form concaves on the silicon substrate.
However, a laser processing time is a problem in this method. Because laser processing corresponding to the area of a concave is performed, a laser processing area must be increased with respect to a surface area to form fine concaves and convexes over the entire surface of the silicon substrate. This requires a long processing time that is far from practical solar-battery production processing and thus the entire processing time has to be extended.
Therefore, there is considered the following method. That is, a dot-shaped small area is processed by the laser processing, etching by a mixed solution of hydrofluoric acid and nitric acid is subsequently performed to form an undercut under a mask, and finally etching by an alkaline aqueous solution is performed, so that the undercut is formed in an inverted pyramid shape. Because the size of a concave shape can be larger than that of the laser processing by the undercut, even when a laser processing diameter is small, a concave-convex shape can be finally laid on the entire surface. As such a method, a method of performing etching by an alkaline aqueous solution subsequent to etching by hydrofluoric acid and nitric acid has been proposed (see, for example, Patent Literature 2).